What is viscosity?
This question is often best answered by example. Imagine a styrofoam cup
with a hole in the bottom. If I then pour honey into the cup I will find
that the cup drains very slowly. That is because honey's viscosity is
large compared to other liquids' viscosities. If I fill the same cup
with water, for example, the cup will drain much more quickly.

Viscosity is a measure of a fluid's resistance to flow. It
describes the internal friction of a moving fluid. A fluidwith large
viscosity resists motion because its molecular makeup gives it a lot of
internal friction. A fluid with low viscosity flows easily because its
molecular makeup results in very little friction when it is in motion.

Gases also have viscosity, although it is a little harder to notice it in
ordinary circumstances.